.travis.yml: only use wheels for numpy

Don't try to use wheel for PyOpenCL, etc. because the build becomes too brittle.
.travis.yml: explicitly install matplotlib and ipython
.travis.yml: do not use wheel when installing dtcwt

The pytools brought in as a wheel from pyopencl appears to be broken in Py3. Work around it by explicitly installing it without using wheel.
Merge pull request #63 from rjw57/fix-py3k-opencl

Fix "wheel-ised" build on travis-ci